1. Field of the Invention
This invention relates to electronic modules which use a thermal paste as the cooling element having improved thermal performance by reducing thermal paste gap tolerances, and to a method for determining and fixing the appropriate thermal paste gap specification for a particular module. In particular, this invention relates to an electronic module utilizing shims to fix the thermal paste gap specification and a method of determining the necessary shim size to be used.
2. Description of Related Art
Integrated circuit chips mounted on substrates and particularly in an array on the substrate present special cooling difficulties. While chip size has increased over the years and power usage is more efficient than in the past, new cooling systems are still needed to remove the relatively high power density generated by the chip. The prior art is replete with different types of thermal conduction modules (TCMs) designed to enclose and cool integrated circuit chips on ceramic glass substrates. In some instances, the TCMs utilize liquid coolants to cool the chips, such as those described in U.S. Pat. Nos. 5,239,200; 5,294,830; and 5,177,667 issued to the assignee of this application. Another approach to cooling has been described in U.S. Pat. No. 4,500,945 also assigned to the assignee of this application by the use of pistons contacting the chips within the TCMs to remove heat.
A further approach in cooling chips in TCMs has been to utilize a thermally conductive medium, such as high thermal conductivity paste, between the top of the integrated circuit chips mounted on the substrate and the lower surface of the cover plate facing the substrate. An example of the useful, stable, high solid content, high thermal conductivity paste is disclosed in U.S. Pat. No. 5,098,609. The compound can be applied as a thin film between the top of the chip and the lower surface of the cover of the TCM. In order to properly control the amount of heat removed from the integrated circuit chip, it is desirable to determine a gap of a specified and desired fixed distance between the top of the chip and lower surface of the cover, and fill that space completely with a thermally conductive medium such as the aforementioned paste.
In modules using thermal pastes, the tolerances that directly affect the paste gap dimension, or distance separating the device from the cooling hardware, must be held below acceptable levels, which are typically quite small, or provide a scheme for adjusting out the tolerances in the design of the hardware. Due to the magnitude of typical tolerances on parameters such as chip thickness, substrate thickness, flatness, machined features, etc., the resultant effect on the paste gap would be unacceptably large without adjustment. Therefore, a method of determining and fixing the paste gap within a much tighter tolerance band increases the thermal performance of the module.
Prior art methods of adjusting the proper paste gap between the top surface of the chip and the lower surface of the cover have utilized an integral space within the thermal conduction module to establish the required thermal gap. This spacer is normally integral with the cover plate and extends around the periphery of the area of the substrate on which the integrated surface chips are mounted.
Glass ceramics which are normally used as substrate materials for the integrated circuit chips are brittle and easy to break and must be handled with care. The substrates are normally mounted in the base plate of the TCM which encompasses the edges of the substrate. The cover plate is normally secured to the base plates surrounding the substrate following assembly of the TCM. It has been found that the use of the integral spacer in the TCM can contribute to breakage of the glass ceramic substrates when the cover plate is fastened too tightly to the base plate. This has been found to occur because the integral spacer transfers force directly to the substrate when the fasteners, typically assembly screws or bolts around the periphery of the cover plate, are tightened down to the base plate. The substrate, which is contained within the base plate by a lip extending within its periphery, is then subject to excessive and uneven forces from the spacer which may not be in direct line with the lip of the base plate. The application of excessive force by the fasteners tend to break the substrates near the lip of the base plate. Because parallelism of the lower surface of the cover plate and the top of the integrated circuit chips must be maintained in all assembled parts, an evenly distributed force must be applied around the periphery of the TCM.
A prior art method used by applicant to adjust and fix the paste gap dimension is the use of a set of 22 adjustable set screws to fasten the cover plate to the base plate. However, it has been found that set screw torque properties may present variables resulting in rejected lots and adjustment uncertainty.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a thermal conduction module having improved thermal and mechanical stability and performance.
A further object of the invention is to provide a thermal conduction module having improved thermal performance which may be assembled with a controlled gap between the lower surface of the cover plate and the upper surface of the integrated circuit chip without causing excessive force and breakage to the chip substrate, whereby a desired thermally conductive medium may be applied between the chip and the cover plate to effect controlled heat removal and cooling of the chip.
A further object of the invention is to provide a method of assembling a thermal conduction module having improved thermal performance.
It is yet another object of the present invention to provide a process for determining the dimension of the thermal paste gap for a particular thermal conduction module.
It is yet another object of the present invention to provide an apparatus for use in fixing the thermal paste gap dimension within a thermal conduction module.
It is yet another object of the present invention to provide a method of making the apparatus used in fixing the thermal paste gap dimension within the thermal conduction module.
It is yet another object of the present invention to provide a kit for fixing the thermal paste gap dimension in electronic modules utilizing thermal pastes as the cooling method.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.